1. Field of the Invention
This invention relates to a process and apparatus for recovery of solvents.
2. Description of the Prior Art
In the known processes and apparatus, which are described, for example, in Ullmann's Encyclopedia of Technical Chemistry, Vol. 1 (1951), page 338, a carrier gas stream laden with solvent vapors in an evaporation space is cooled to condense out solvent vapors and to separate the solvent. After condensation and separation the carrier gas stream lean in solvent vapors is returned into the evaporation space after being reheated. The solvent vapors are not completely condensed out of the carrier gas stream since there remains in the carrier gas stream a certain residual amount of solvent vapor, corresponding to the vapor pressure of the solvent at the temperature of the coolant. In order to avoid losses of solvent, the carrier gas stream is conducted in a circuit. The ability of the carrier gas stream, which is lean in solvent vapors, to take up solvent vapors is hence in fact somewhat reduced; however, this is unimportant for the efficiency of the process.
The process is quite generally suitable for removal of volatile solvents from non-evaporable substrates.
A field of application is the removal of solvent residues from chemical substances which have been produced or purified with the use of solvents. Further fields of application lie in the paint and lacquering fields, the field of chemical cleaning of textiles, the film and foil field, the rubber processing field, and the adhesives and adhesive materials field.
In prior art apparatus, generally, separate cooling devices for condensing out the solvent vapors and devices for reheating the carrier gas stream lean in solvent vapors are provided. Deficiencies in these prior art apparatus include that considerable amounts of coolant are required and a high energy demand to reheat the carrier medium lean in solvent vapors. This reheating of the carrier gas stream is necessary so that the carrier gas stream can rapidly become re-laden in the evaporation space with a sufficient amount of solvent vapor, i.e., so that the substrate is rapidly dried.
It was possible to attempt to achieve a saving of energy by using the coolant which has been heated on passage through the cooling device for reheating the carrier gas stream lean in solvent vapors, i.e., to conduct the coolant countercurrent to the carrier gas stream. However, it is immediately obvious that only a small fraction of the heat taken from the carrier gas stream previously in the cooling device can be returned to the carrier gas stream. Because of the relatively low temperature difference between the carrier gas and the coolant, the cooling device and the device for reheating the carrier gas stream have to be provided with large heat exchange surfaces.
The process is thus disadvantageous not only because of its high energy and coolant consumption, but also because of its high cost in apparatus.
An apparatus for recovery of solvent from a hot carrier gas stream laden with solvent vapors is known from DE-PS No. 27 25 252, in which the carrier gas stream is compressed, cooled, and expanded with production of work, for condensing out the solvent vapors and separating the solvent. The carrier gas stream lean in solvent vapors is conducted back into the evaporation space after being reheated.
Return of this carrier gas stream takes place, however, in admixture with a carrier gas stream taken from the evaporation space and laden with solvent vapors. After being heated in indirect heat exchange with the compressed carrier gas stream, the admixture is conducted back into the evaporation space in a duct loop together with the carrier gas stream lean in solvent vapors. Better heat regulation is accomplished but having the disadvantage that the carrier gas stream conducted back into the evaporation space has a relatively high content of solvent vapors. The drying effect in the evaporation space is reduced in this manner. It is further disclosed in DE-PS No. 27 25 252 that the work liberated by the expansion in an expansion turbine can be recovered. However, details are lacking as to where this work can be usefully utilized.
An object of the present invention is to improve upon a process and an apparatus of the kind described above, with low cost in apparatus while utilizing the work produced from the expansion of the compressed carrier gas stream laden with solvent vapors and the carrier stream returned to the evaporation space is as lean as possible in solvent vapors.